Conventionally-known color image reading apparatuses are mostly of the type that reads an original image by forming a reduced image thereof on an image sensor (for example, a Charge-Coupled Device (CCD)) through a focusing lens system. Such a color image reading apparatus typically employs a color image reading lens system composed of five or six lens elements that offers magnifications ranging from -1/4.times. to -1/10.times.. Moreover, color image reading apparatuses proposed in Japanese Published Patent Application No. H7-121047, Japanese Laid-open Patent Application No. H4-73711, and others are designed to perform color separation by means of a diffraction grating. Some commercially-available digital copiers are provided with a color-separating image reading optical system composed of seven lens elements and a reflection-type diffraction grating.
However, in the conventional color image reading apparatuses described above, a diffraction grating is employed merely as a color-separating element, and no arrangement has ever been known that takes advantage of the light-diffracting ability of a diffraction grating to correct aberrations.